Acids and bases are important intermediates in the manufacture of a large number of chemicals. After they have been used, these acids and these bases generally are in the form of saline aqueous solutions from which it is necessary to strip them. For environmental and economic reasons, it is desirable to regenerate the initial acids and bases directly from the salts contained in these industrial effluents.
Electrodialysis using bipolar membranes enables such regeneration to be carried out. This known method uses electrical energy to dissociate the water of the saline solution and to recover the acid and the base separately according to the reaction: ##STR1##
In order to carry out this reaction and to keep the species separate, ion-exchange membranes and, more particularly bipolar membranes consisting of two faces, respectively selective to the anions and to the cations, are used. Under the influence of an electric field, these membranes allow the following reaction: EQU H.sub.2 O.fwdarw.H.sup.+ +OH.sup.-
The H.sup.+ +OH.sup.- ions are then recombined respectively with the anions X.sup.- and cations M.sup.+ coming from the salt, and the species obtained are kept separate by conventional (monopolar) ion-exchange membranes in a three-compartment cell.
This method of regeneration from salts into acids and into bases has already been applied to many cases, for example:
sulfuric acid from Na.sub.2 SO.sub.4 (the Patent Publication JP-4-132605 and the article by S. SRIDHAR "Elektrodialyse mit bipolaren Membranen [Electrodialysis using bipolar membranhes]" in Chem. Ing. Tech. 61 (1989) No. 5, pp. 428-429); PA1 hydrochloric acid from NaCl [abstracts Chemical Abstracts 117(16): 153850m, 109(2): 11524u, 92(6): 43961f and 92(4): 25083s]; PA1 butyric acid from its sodium salt [abstract CA 116(18): 182124n]; PA1 maleic acid from its ammonium salt (the aforementioned article by S. SRIDHAR); PA1 boric acid from borates [abstracts CA 107(4): 29755p and CA 114(4): 30673m]; PA1 tartric acid from its potassium salt (Patent Application FR 2,646,421); PA1 organic sulfonic acids [abstract CA 71(12) 56451f and U.S. Pat. No. 5,221,443].
The bipolar membranes currently available on the market exhibit performance characteristics which vary depending on the technology used to manufacture them and depending on the supplier. Owing to their nature, bipolar membranes are in principle nonpermeable to cations and to anions which are respectively stopped by the anionic and cationic layers of the bipolar membrane.